Dept. of Mechatronics Engg.

BIOMEDICALINSTRUMENTATION

Biomedical Amplifiers

Dr. Mohsin Tiwana

mohsintiwana@gmail.comFeedback and stability

 A negative feedback system (left) with closed loop gain G1G2 > 1 and no phase shift in the loop will always be stable since we are just subtracting in-phase components from the input and amplifying. Q: What if we change the feedback to positive? (right) A: Now we are taking a signal from the output, ADDING it to the input and amplifying it → UNSTABLE!!Feedback and stability

 Q: What if we have NEGATIVE feedback, high closed loop gain G1G2, but a phase shift of 180o in the amplifier? A: The negative feedback becomes POSITIVE feedback and we get INSTABILITY! Most modern opamps are stable when used as designed but if we are creating our own feedback loops with RLD circuits we must be careful. Methods of preventing negative feedback instability:  Reduce phase shift in the loop  Reduce the loop gain at the frequency where the phase is 180o. Op-amps +VBB The basic building block of amplifiers used to measure biopotentials is the operational amplifier (op-amp or opamp) RL RL A simplified circuit of a BJT (bipolar junction transistor) op-amp front end is shown right, together with the op-amp’s IB1 IB2 circuit symbol VOUT Ideal opamp Q1 Q2  A =  (infinite gain)  v+ = v- (no offset voltage) Vs1 RC Vs2  Rd =  (infinite input impedance)  Ro = 0 (zero output impedance)  BW = (infinite bandwidth) -VBB (a)  Ф = 0 (zero phase shift) v1 No opamp is ideal – we must design the - VOUT circuit to account for the characteristics of the opamp used v2 + (b) Non-ideal opamps GAIN  Typically ~ 100,000 at dc.  Several stages, each of which has stray or junction capacitance → the gain falls off as the frequency increases.  As an amplifier, the op-amp is never used in open-loop mode. It always has negative feedback around it.  Op-amps such as the 741 have a unity-gain bandwidth of about 1 MHz, but for high frequency applications op-amps with bandwidths of >100 MHz are available. OFFSET VOLTAGE  The two input voltages v+ and v- may differ by a few mV.  When amplifying small signals the offset voltage may need to be nulled out.  Alternatively you can use multiple gain stages with AC coupling to remove the offset.  Or find a precision op-ampNon-ideal opamps SLEW RATE  The frequency response referred to above, is for small signal inputs.  For large signals, there is an additional limitation. When rapid changes in outputs are demanded, the compensation capacitor must be charged up from an internal source that has limited current capacity.  The rate of change in voltage across the capacitor is then limited, and as a consequence the rate of change of the output voltage is limited to a maximum slew rate – often of the order of 1 volt per microsecond.  Slew rate is not normally an issue for biological signals which have fairly long rise times, even when using micropower opamps (low slew rate)Non-ideal opamps INPUT RESISTANCE  For the typical BJT op-amp, this is about 0.5 meg ohms. However, the amplifier itself, because of feedback, will exhibit a much higher input resistance.  For a voltage follower, it is A times as great. For an inverting amplifier, it is equal to the external input resistor value. In some applications, where extremely high input resistances are required, FET-input op-amps are useful.

 OUTPUT RESISTANCE  Typically, this is about 100 ohms, but because of feedback in the overall amplifier, its value is reduced by a factor A and so becomes negligible for most applications.Non-ideal opamps

 NOISE  Low frequency (drift) noise is generated when temperature variations cause the offset voltage to vary.  At more cost, op-amps with tighter drift specification (eg 0.1V/C) are available than ordinary low-cost types.  Temperature drift is usually not a big issue for biomedical instrumentation which is usually used in office conditions (constant temp) BIAS CURRENT  Base current must flow all the time to keep the input transistors turned on  The error is minimized by connecting, at the non-inverting input, a resistor equal to the parallel resistance to ground connected to the inverting input (i.e. the input and feedback resistors).  There will still be a small error remaining because of the slight difference between the bias currents of the two input transistors (called the input offset current in the specifications). Basic amplifier circuits Inverting amplifier Vo    R2 / R1  Va R2  Input impedance = R1 R1 va - vo +

v2 + With buffered inputs:  eliminates the influences of electrode impedances on input impedance and their imbalance on CMRR  R1 and R3 have no influence on input impedance

 Still requires matched resistors for good CMRR

Instrumentation amplifier

2 R1 R3 AD  (1  ) Rgain R2

 Advantages  varying Rgain does not change the common mode signal hence CMRR increases in direct proportion to differential gain (a useful property);  If V+ = V-, 0 V across Rgain, and ACM = 1 (in first stage, see later).  Large CM voltages can be handled  if the two input amps are similar, their common-mode errors tend to be cancelled by the 3rd amp due to symmetry. Disadvantages  High parts count but can be bought as a single IC Typical instrumentation amp VD V1  VCM  2 + V1 '

VD VD VCM  VCM VCM  2 2 Current flowing VD  R1  V1 '  Vcm  1   and VD VD 2  R 2  Vcm   Vcm  2 2 VD VD  R1  I  V2 '  Vcm  1   2 R2 2 R2 2  R 2 Instrumentation amplifier CMRR Since VCM is not amplified, best strategy is to maximise CMRR by maximising gain of first stage  R  1  1   R2  HOWEVER, AAMI says we need to cope with offsets of ± 300mV on any lead. So this limits the gain, depending on our power supplies and capacities of amp, e.g. ± 5V rails we should not use a gain much higher than 10 (may cause saturation) Note the following stage is just a common differential amplifier (with gain of R4/R3)  Output  R1  R4 VO / VD VO  VD 1   CMRR    R2  R3 VO / VCM We can either use the potentiometer to get high CMRR or use precision resistors. A gain of 10 would add 20dB to CMRR attainable just from a differential amplifier. AC instrumentation amplifier DC offsets can be a problem – lose high CMRR Gain(magnitude) Place a capacitor in series with R2. DC gain of front stage is 1 (for differential and common mode). This means we can make front end gain as high as we like without 1 worrying about offset voltages. In turn leads to very high CMRR But since R2 is small for high gain means we need to use very high f capacitances for low fc (0.05Hz). fc=1/(2πR2C) Capacitances this high are only available as polarised, electrolytic capacitorsPatient Protection

 Dangerous currents are not allowed to flow into the

patient, by putting current limiting resistors in the patient leads  The high impedance input (>10MΩ) guarantees that in normal operation this doesn’t lead to a reduced input  In the case of the amplifier breaking down (single fault condition) and shorting the input to the power supply, the current is limited by the resistors  10KΩ is usually sufficient As well as protecting the patient, it is often desirable to protect the equipment from defibrillator or other high voltage inputs. This is normally done using diodes or breakdown devices (e.g. neon tubes)  See next slide  The patient current limiting resistors above also protect the deviceDefibrillation protection

10K, 3000V

10K

 The first resistor must be able to withstand very high voltages for short periods of time (~10msec)  Example: carbon composition type resistor (NOT film type) The second resistor limits current into the amplifier (which has its own internal protection diodes) The gas discharge tube in middle can be replaced by zener diodes  It clamps the midpoint voltage to a few hundred volts The resistors also protect the patient against fault conditions and the amp against RF interference Isolation amplifiers

 An Power isolation amplifier is an Power instrumentation Supply amp that has its Supply signal input circuit isolated from the power input and signal outputut circuits (2-port isolator). Output Input Output

 A 3-port isolator also has isolation

between power input and signal output. 2-port  They provide extra CMRR and patient protection. Power  Transfer of signal and power Supply across the isolation barriers is via optical coupling or magnetic coupling. Input Output

 As they tend to be expensive,

isolation in commercial devices is often put in digitally later in the circuit, especially for multi-channel 3-port devices.Movement artefact This can induce quite large voltages on the patient, as common mode voltages, and transiently as differential voltages. It is therefore desirable not to have DC amps unless the biopotential requires it (e.g. EOG). However, putting coupling capacitors in the differential pathway is likely to degrade the CMRR because  (i) of the difficulty in matching C’s (leads to potential divider effect)  (ii) the need to provide input bias current for the op-amps – i.e. dc path to common on both + and – inputs (not a problem for high input impedances >10MΩ) Usually, differential input stages are DC coupled, while coupling capacitors only used when single-ended. If the gain is high before this, the capacitor might “block” (i.e. only discharge slowly) if an amplifier saturates, thus ceasing output for some time. The early differential stages are thus invariably designed with low gain until the signal is single-ended (after the instrumentation amp)Sample design Driven right leg details

Iids RRLR out Right leg driven (RLD) circuit Ra/2 Rf KCL at opamp -ve input id 2 v c m / Ra vo / Rf 2Vcm / Ra  V0 / R f  0 + - Ro  V0  2 R f Vcm / R a , but vo vc +  Vcm  R RL id  V0 , thus mvc m RRL id  Vcm  RRL (1  2 R f / Ra ) id RRL id  Vcm  Example: with reference to the figure (1  G ) determine the common mode voltage where G  2 R f / Ra  gain of RLD cct Vcm on the patient when a displacement current of id = 0.2 A flows to the The effective resistance between the right leg patient from the power lines. Choose and ground is thus resistance values so that the common mode voltage is minimal and there is only a high resistance path to ground R e f f  RRL /(1  G) when the drive op-amps saturates. Designing RLD When the differential amplifier saturates the saturation voltage appears at the input to the drive circuit and could result in a high voltage at the right leg electrode.  Note due to high gain of RLD cct that it will saturate before the differential amplifier does When the RLD amp is saturated the normal laws of feedback no longer apply. Under such conditions VCM increases and there is increased current flow to the subject. Ro is thus usually included to limit any current to safe levels (from 10KΩ to 5MΩ depending on other isolation). However, when the amplifier is not saturated, it is desirable to make the common mode voltage as low as possible. Hence Ro is usually included in the feedback loop To minimise common mode voltage Vcm requires the ratio Rf/Ra to be large. Rf may be as large as 5MΩ and typically Ra is around 25 KΩ giving a loop gain of 400. The effective resistance between right leg and ground Reff is then (if RRL = 20KΩ):

R e ff  RRL /(1  G ) VCM  50  0.2  A

 20000 / 401  10 V  50RLD (continued)

 If the loop gain G is high and sufficient phase shift occurs, instability and oscillations can result.  Note other phase shifts in circuit affect this, such as input lead shield capacitance. This can be partly compensated for by replacing Rf by a capacitor (next slide), and ensuring that the amp is well isolated and has a low leakage capacitance to ground. So far we have only considered 50 Hz of mains CM voltage.  But fluorescent lights can cause a CM voltage as a short burst of 1kHz of radiation at 10ms intervals which depending on the patients position and other factors, could be as large as 10-50% of the 50Hz CM voltage.  This high frequency interference can be transformed into 100 Hz interference by non-linearities in the electronics or recorder. The driven right leg circuit, provided it has sufficient gain at 1 kHz will also reduce this noise signal to an acceptable level. Overall front end for ECG amp To avoid instability, loop - Input a gain of driven shield + circuit is made < 1 10k 100 - - + Loop gain of the driven +

RL circuit is 300 at 50 Hz,

giving a 50 dB Input b -

improvement in CMRR +

 The gain is lower at 1nF 10k

higher frequencies to 1M -

avoid oscillations Driven Right +

Leg Anatomical Planes An anatomical plane is a structure used to transect the human body, in order to describe the location of structures or the direction of movements. In human and animal anatomy, three basic planes are used: The sagittal plane, being a plane parallel to the sagittal suture, divides the body into sinister and dexter (left and right) portions.  The midsagittal or median plane is in the midline; i.e. it would pass through midline structures such as the navel or spine  The coronal or frontal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions. The transverse plane, also known as an axial plane or cross-section, divides the body into cranial and caudal (head and tail) portions.Electrocardiogram (ECG)

 A beating heart generates an electrical signal

 This appears throughout the body and on the surface  Can be used as a diagnostic tool to assess cardiac function  or simply to monitor that the heart is beating adequately. For diagnostic purposes, twelve ECG traces are usually recorded. These are:  Three limb leads – leads I, II and III.  Three augmented limb leads – aVR, aVL, and aVF.  Six chest leads – V1 to V6. Why so many?  When a cardiac ischaemia (lack of oxygen) occurs it can scar the tissue and perhaps lead to an infarction (dead tissue)  These conditions can change the way the ECG appears but it may only show up in one or two leads, depending where it occurs.Einthoven’s triangle

 The voltages of leads I, II and III can be considered to be projections of the equivalent cardiac dipole on an approximately equilateral triangle in the frontal plane – Einthoven’s triangle The bipolar limb leads add vectorially: II = I + III I = VLA – VRA, II = VLL – VRA, III = VLL – VLAECG Limb leads

Note additive effects

Dipoles

 Fixed origin dipoles in 3 dimensions require 3 variables to

describe (e.g. magnitudes in x, y and z directions) We can consider the ECG to arise from the cardiac vector, which is a 3 dimensional vector changing in amplitude and direction during the cardiac cycle. The bipolar limb leads are measuring the projections of this dipole (during time) in the 3 lead directions 0o, 60o and 120o (see previous slide) in the frontal planeGeneration of the ECG signal in theEinthoven limb leads At the start of the cardiac cycle the dipole begins at the sinoatrial (SA) node and is small in magnitude Voltages measured between two body surface electrodes (lead voltage) depend on:  lead location  heart location  heart vector (position, direction and magnitude)  torso volume inhomogeneitiesGeneration of the ECG signal in theEinthoven limb leadsGeneration of the ECG signal in theEinthoven limb leads Lead vector

 Frank Norman Wilson (1890-

1952) investigated how ECG unipolar potentials could be defined Wilson central terminal (WCT) was formed by connecting equal-valued resistors from each limb lead to a common node Voltage at WCT is the average of the voltages at the 3 limb electrodes  provides reference potential for unipolar measurement Modern equipment does not determine this by an electrical circuit but mathematically.Augmented limb leads The limb electrode voltages can form so-called unipolar rather than bipolar leads.  For example, WCT could be used as a reference for each. In 1942 Goldberger noted that if the average of the other two limb leads were used instead of WCT as the reference, the signal became 50% larger  Hence the term “augmented” aVR, aVL and aVF But they contain the same information as the bipolar leads and can be considered redundant Precordial (chest) Leads

 Pre-cordial (in front of, the heart)

 In addition to the six limb leads (I, II, III, aVR, aVL, aVF), a 12-lead ECG includes six “chest” leads. The chest leads look at the heart’s electrical activity in a slightly off- horizontal plane around the front of the chest (traverse plane). This detects problems that might not be obvious from the standard limb leads, which measure electricity in a vertical (frontal) plane. The chest leads are often called V-leads.European vs. American 12 lead ECG AHA North America IEC Europe

Inscription Colour Location Inscription Colour

RA White Right arm R Red

LA Black Left arm L Yellow

RL Green Right leg N Black

LL Red Left leg F Green

V1-6 Brown Chest C1-6 White

Information content of 12 lead ECG Much redundant information 12 leads (traces) ECG requires only 8 A/D channels (why?) We really only need 3 leads (x, y and z) to measure in 3 planes Vectorcardiography (VCG) The basic principle of vectorcardiography is illustrated based on ideal uniform lead fields which are mutually orthogonal being set up by parallel electrodes on opposite sides of the torso (bipolar configuration) Projection of Heart Vector into 3 planes – frontal, traverse and sagittal Track change in 3D vector co-ordinates over time course of cardiac cycle Frank lead system The lead matrix of the Frank VCG-system. The electrodes are marked I, E, C, A, M, F, and H, and their anatomical positions are shown. The resistor matrix results in the establishment of normalized x-, y-, and z- component lead vectors References Cardiovascular Physiology Concepts  http://www.cvphysiology.com/Arrhythmias/A013a.htm